Methods of fabricating semiconductor devices with enlarged recessed gate electrodes

ABSTRACT

A semiconductor device includes a semiconductor substrate having a recess therein. A gate insulator is disposed on the substrate in the recess. The device further includes a gate electrode including a first portion on the gate insulator in the recess and a second reduced-width portion extending from the first portion. A source/drain region is disposed in the substrate adjacent the recess. The recess may have a curved shape, e.g., may have hemispherical or ellipsoid shape. The source/drain region may include a lighter-doped portion adjoining the recess. Relate fabrication methods are also discussed.

REFERENCE TO PRIORITY APPLICATION

The present application is a Divisional Application of U.S. patentapplication Ser. No. 11/556,352, filed in the United States PatentOffice on Nov. 3, 2006 now U.S. Pat. No. 7,541,656, which is adivisional of U.S. patent application Ser. No. 10/738,316, filed in theUnited States Patent Office on Dec. 17, 2003 now U.S. Pat. No.7,148,527, which claims priority to Korean Patent Application No.2002-81091, filed in the Korean Intellectual Property Office on Dec. 18,2002. The disclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to semiconductor devices and methods offabrication therefor, and more particularly, semiconductor devices withrecessed gate electrode portions and methods for forming the same.

An integrated circuit (IC) may include several metal oxide semiconductor(MOS) transistors. As ICs become more highly integrated, the scale ofsuch MOS transistors may decrease. Therefore, there is a need forreducing problems arising from short channel effects for suchsmall-scale MOS transistors.

A gate of a MOS transistor may be formed by a method wherein a gateinsulator is formed on a semiconductor substrate and then a gateconductive layer is formed on the gate insulator and patterned.Source/drain regions may be formed by performing ion-implantationprocess after forming the gate electrode.

However, the gate formed by a conventional patterning process may haveetch damage on sidewalls thereof, so that charge may inconstantly flowfrom the channel to the periphery of the gate and leakage of current mayoccur. Additionally, conventional methods may not be capable of forminga gate electrode having a shorter channel length than a resolution of aphotolithography process. This may serve as a limit on operation speedand power consumption characteristics.

In order to solve problems due to etch damage and to improvecharacteristics of a semiconductor device, a T-shaped gate electrodeformed by a damascene process has been introduced as shown, for example,in U.S. Pat. No. 6,255,202.

FIGS. 1 through 4 are cross-sectional views showing a conventionalmethod for forming a T-shaped gate electrode. Referring to FIG. 1, anoxide layer 13 and a nitride layer 15 are sequentially formed on asemiconductor substrate 11. The nitride layer 15 is patterned using aphotolithography process, thereby forming an initial groove 17 exposingthe oxide layer 13. Referring to FIG. 2, spacers 19 are formed onsidewalls of the initial groove 17. The exposed oxide layer 13 ispatterned using the spacers 19 as etch masks, thereby exposing thesemiconductor substrate 11.

Referring to FIG. 3, the spacers 19 are removed to form a final groove21. A gate oxide layer 23 is formed on the exposed semiconductorsubstrate 11, and the final groove 21 is filled with a conductivematerial, thereby forming a T-shaped gate electrode 25. Although notillustrated, the nitride layer 15 is removed and an ion-implantationprocess is performed to form source/drain regions (not shown) in asubsequent process.

In order to reduce problems arising from a short channel effect,source/drain regions are typically formed very shallowly. However, assemiconductor devices become increasingly highly integrated, it isgenerally more difficult to form source/drain regions very shallowly ina process of forming a T-shaped gate or a conventional gate. To addressthese problems, a process of forming a recessed gate electrode portion(or an elevated source/drain) has been introduced, as described, forexample, in U.S. Pat. No. 6,303,448, which discloses a process forforming a recessed gate electrode portion (or an elevated source/drain).

FIGS. 5 through 7 are cross-sectional views showing a conventionalmethod of forming a recessed gate electrode portion. Referring to FIG.5, an oxide layer 53 and a nitride layer 55 are sequentially formed on asemiconductor substrate 51. The nitride layer 55, the oxide layer 53 anda part of the semiconductor substrate 51 are anisotropically dry-etchedto form a groove 57. Referring to FIG. 6, a gate insulator 63 is formedon the surface of the etched semiconductor substrate 51 by a thermaloxidation process. Referring to FIG. 7, the groove 57 is filled with aconductive material, thereby forming a recessed gate electrode portion65. Although not illustrated, the nitride layer 55 is removed and anion-implantation process is performed to form source/drain regions in asubsequent process.

The conventional methods described above may have several problems.According to a conventional process of forming a T-shaped gateelectrode, it may be difficult to form source/drain regions with shallowjunctions. This may be because the channel typically is formed around asurface of a semiconductor substrate like a conventional gate process.Also, drain-induced barrier lowering DIBL may occur, which may cause aleakage current between the semiconductor substrate and a gate insulatorat a lower voltage than a threshold voltage.

According to some conventional processes of forming a recessed gateelectrode portion, a capacitance of a parasitic capacitor is increaseddue to overlap of gate electrode and source/drain. This is because thesemiconductor substrate is partially etched to enlarge a contact areabetween the gate electrode and the semiconductor substrate and also agate insulator of a thermal oxide is thinly conformally formed along aninner wall of a groove formed on the semiconductor substrate.Gate-induced drain leakage (GIDL) may be increased in a devicefabricated in such a manner.

SUMMARY OF THE INVENTION

According to some embodiments of the present invention, a semiconductordevice includes a semiconductor substrate having a recess therein. Agate insulator is disposed on the substrate in the recess. The devicefurther includes a gate electrode including a first portion on the gateinsulator in the recess and a second reduced-width portion extendingfrom the first portion. A source/drain region is disposed in thesubstrate adjacent the recess. The recess may have a curved shape, e.g.,may have a hemispherical or semielliptical shape. The source/drainregion may include a lighter-doped portion adjoining the recess.

In some embodiments, the gate insulator comprises a first portiondisposed on a sidewall of the recess and having a first thickness, and asecond portion disposed on a bottom of the recess and having a secondthickness less than the first thickness. The first portion of the gateinsulator may adjoin a source/drain region in the substrate.

In further embodiments of the present invention, the device furtherincludes an insulation layer on a surface of the substrate adjoining thesecond portion of the gate electrode above the substrate and extendingover a portion of the first portion of the gate electrode. An insulatingspacer is disposed on a sidewall of the second portion of the gateelectrode and on the insulation layer.

In further embodiments of the present invention, the gate electrodefurther comprises a third portion on the second portion, the thirdportion having a greater width than the second portion. An insulationlayer on a surface of the substrate may adjoin the second portion of thegate electrode above the substrate and extend over a portion of thefirst portion of the gate electrode, and an insulating spacer may bedisposed on a sidewall of the second portion of the gate electrode, on asidewall of the third portion of the gate electrode and on theinsulation layer. In some embodiments, the gate insulator may include afirst portion disposed on a sidewall of the recess and having a firstthickness, and a second portion disposed on a bottom of the recess andhaving a second thickness less than the first thickness. In otherembodiments, the gate insulator comprises a substantially uniformthickness insulation layer lining the recess.

In some method embodiments of the present invention, a semiconductordevice is formed. An insulation layer is formed on a substrate, and asacrificial layer is formed on the insulation layer. An opening throughthe sacrificial layer and the insulation layer is formed to expose anactive region of the substrate. A portion of the active region isremoved to form a recess (e.g., having a hemispherical, semiellipticalor other curved shape) therein that has a greater width than the openingthrough the sacrificial layer and the insulation layer. A gate insulatoris formed in the recess and a gate electrode is formed extending throughthe sacrificial layer and the insulation layer and into the recess. Thegate electrode includes a first portion on the gate insulator in therecess and a second reduced-width portion extending from the firstportion. The sacrificial layer is removed to expose a sidewall of thegate electrode outside of the recess, and an insulating spacer is formedon the exposed sidewall of the gate electrode. A source/drain region isformed in the substrate adjacent the recess.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 4 are cross-sectional views showing a conventionalmethod of forming a T-shaped gate electrode.

FIGS. 5 through 7 are cross-sectional views showing a conventionalmethod of forming a recessed gate electrode portion.

FIG. 8 is a cross-sectional view showing a semiconductor deviceaccording to some embodiments of the present invention.

FIG. 9 is a cross-sectional view showing a semiconductor deviceaccording to further embodiments of the present invention.

FIG. 10 is a cross-sectional view showing a semiconductor deviceaccording to still further embodiments of the present invention.

FIGS. 11 through 25 are cross-sectional views showing operations forforming the semiconductor device of FIG. 8.

FIGS. 26 through 31 are cross-sectional views showing operations forforming the semiconductor device of FIG. 9.

FIGS. 32 through 36 are cross-sectional views showing operations forforming the semiconductor device of FIG. 10.

FIGS. 37 through 40 are cross-sectional views showing operations forforming a semiconductor device according to additional embodiments ofthe present invention.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, the thickness of layers and regions are exaggerated forclarity. It will be understood that when an element such as a layer,region or substrate is referred to as being “on” another element, it canbe directly on the other element or intervening elements may also bepresent. Furthermore, relative terms, such as “beneath,” may be usedherein to describe one element's relationship to another element asillustrated in the Figures. It will be understood that relative termsare intended to encompass different orientations of the device inaddition to the orientation depicted in the figures. For example, if adevice in the figures is turned over, elements described as “below”other elements would then be oriented “above” the other elements. Theexemplary term “below,” therefore, encompasses both an orientation ofabove and below.

It will be understood that although the terms first and second are usedherein to describe various regions, layers and/or sections, theseregions, layers and/or sections should not be limited by these terms.These terms are only used to distinguish one region, layer or sectionfrom another region, layer or section. Thus, a first region, layer orsection discussed below could be termed a second region, layer orsection, and similarly, a second without departing from the teachings ofthe present invention. Like numbers refer to like elements throughout.

FIG. 8 is a cross-sectional view showing a semiconductor device 800according to some embodiments of the present invention. Referring toFIG. 8, an active region electrically isolated by a field oxide 113 isdefined on a semiconductor substrate 111. The semiconductor substrate111 includes a recess 125 r. A surface of the recess 125 r has asmoothly curved shape. For example, the recess 125 r may have asemielliptical or hemispherical shape.

A gate electrode 135 fills the recess 125 r and has a predeterminedheight. An insulator 132 is interposed between the semiconductorsubstrate 111 and a portion of the gate electrode 135 filling the recess125 r, i.e., a recessed gate electrode portion portion 135 r on asurface of the recess 125 r. The recess 125 r may have a thickness of atleast about 300 Å, for example.

The gate electrode 135 may include the recessed gate electrode portionportion 135 r filling the recess 125 r and a protruding gate electrodeportion 135 p protruding from the semiconductor substrate 111 andconnected to the recessed gate electrode portion portion 135 r. Theprotruding gate electrode portion 135 p may include a bottom gateelectrode portion portion 135 b and a main gate electrode portionportion 135 m. The bottom gate electrode portion portion 135 b has ashorter length than a width of the recess 125 r. The main gate electrodeportion portion 135 m is connected to the bottom gate electrode portionportion 135 b. The main gate electrode portion portion 135 m has alonger length than a width of the recess 125 r. The main gate electrodeportion portion 135 m may include a main gate electrode center portionportion 135 c on the bottom gate electrode portion portion 135 b and amain gate electrode portion silicide spacer 137 on sidewalls of the maingate electrode center portion portion 135 c. An end of the recess 125 ris located between an outer end of the silicide spacer 137 and an outerend of the bottom gate electrode portion portion 135 b. The gateelectrode thus has a shape of an anchor. The recessed gate electrodeportion portion 135 r, the bottom gate electrode portion portion 135 band the main gate electrode center portion portion 135 c may be formedof polysilicon.

In this specification, the gate length indicates a size of a gateelectrode along a direction of a channel. The silicide spacer 137 is fordecreasing resistance of the gate electrode and may be formed by a CVDmethod.

The insulator 132 interposed between the recessed gate electrode portionportion 135 r and the semiconductor substrate 111 may include a buffergate insulator 129 a and a gate insulator 131 of different thicknesses.The buffer gate insulator 129 a is located on sidewalls of the recess125 r. The gate insulator 131 is thinner than the buffer gate insulator129 a and connected to the buffer gate insulator 129 a on a bottom ofthe recess 125 r. Preferably, the gate insulator 131 is arranged on abottom of the bottom gate electrode portion portion 135 b, that is, thebuffer gate insulator 129 a preferably is located on both sidewalls ofthe recess 125 r of both sidewalls of the bottom gate electrode portionportion 135 b. The thick buffer gate insulator 129 a serves to decreasea parasitic capacitance between the gate electrode 135 and thesemiconductor substrate 111 (i.e., source/drain). The thin gateinsulator 131 selves as an effective gate insulator. Thus, a shortchannel transistor may be provided without increase of a parasiticcapacitance by properly controlling thicknesses of the buffer gateinsulator 129 a and the gate insulator 131.

A buffer insulator 115 a surrounds the bottom gate electrode portionportion 135 b of the gate electrode 135 and is aligned with the outerend of the silicide spacer 137. Also, a gate insulation spacer 141 isdisposed on outer sidewalls of the silicide spacer 137 and the bufferinsulator 115 a. The buffer insulator 115 a and the gate insulationspacer 141 isolate the gate electrode 135.

A low concentration impurity-doped region 139 is disposed in thesemiconductor substrate 111 under the buffer insulator 115 a and thegate insulation spacer 141. A high concentration impurity-doped region143 is disposed in the semiconductor substrate 111 out of the gateinsulation spacer 141 and connected to the low concentrationimpurity-doped region 139. The impurity-doped regions 139 and 143 may besource/drain regions. As described above, a parasitic capacitanceoccurring due to an overlap of the gate electrode and the source/drainmay be decreased due to the thick buffer gate insulator 129 a. Silicidelayers 147 may be disposed on the gate electrode 135 and on thesemiconductor substrate 111 out of the gate insulation spacer 141, i.e.,on the high concentration impurity-doped region.

FIG. 9 is a cross-sectional view showing a semiconductor device 900according to further embodiments of the present invention. The device900 is similar to the semiconductor device 800 of FIG. 8, with likeelements indicated by like reference numerals. The device 900 of FIG. 9differs from the device 800 of FIG. 8 in that a gate electrode 135′ ofthe device 900 does not include a silicide spacer.

Referring to FIG. 9, a gate electrode 135′ includes a recessed gateelectrode portion portion 135 r′ filling a recess 125 r of asemiconductor substrate 111 and a protruding gate electrode portion 135p′ connected to the recessed gate electrode portion portion 135 r′. Theprotruding gate electrode portion portion 135 p′ includes a bottom gateelectrode portion portion 135 b′ and a main gate electrode portionportion 135 m′. The bottom gate electrode portion 135 b′ portionconnects to the recessed gate electrode portion portion 135 r′ and has agate length that is less than a width of the recess 125 r. The main gateelectrode portion portion 135 m′ connects to the bottom gate electrodeportion portion 135 b′ and has a gate length greater than a width of therecess 125 r.

Similar to the semiconductor device 800 of FIG. 8, an insulator 132 isinterposed between the recessed gate electrode portion 135 r′ and thesemiconductor substrate 111. The insulator 132 is composed of a buffergate insulator 129 a and a gate insulator 131 of different thicknesses.The buffer gate insulator 129 a is thicker than the gate insulator 131and located on sidewalls of the recess 125 r. The gate insulator 131 islocated on a bottom of the recess 125 r. The protruding gate electrodeportion 135 p′ of the gate electrode 135′ is T-shaped. The protrudinggate electrode portion 135 p′ and the recessed gate electrode portionportion 135 r′ have an anchor shape.

A nitride liner 130 a may be disposed on sidewalls of the bottom gateelectrode portion portion 135 b′ and on the buffer gate insulator 129 a.The device 800 further includes a gate insulation spacer 141 and abuffer insulator 115 a. The gate insulation spacer 141 is disposed onsidewalls and a bottom surface of the main gate electrode portionportion 135 m′. The buffer insulator 115 a surrounds the bottom gateelectrode portion portion 135 b′. The buffer insulator 115 a is alignedwith the gate insulation spacer 141. The gate insulation spacer 141 isL-shaped.

Similar to the semiconductor device of FIG. 8, a low concentrationimpurity-doped region 139 is disposed in the semiconductor substrate 111under the gate insulation spacer 141 and the buffer insulator 115 a. Ahigh concentration impurity-doped region 143 is disposed in thesemiconductor substrate 111 under the gate insulation spacer 141. Thegate electrode 135′ may be formed of polysilicon.

FIG. 10 is a cross-sectional view showing a semiconductor device 1000according to still further embodiments of the present invention. Thedevice 1000 is similar to the device 900 of FIG. 9, with like itemsindicated by like reference numerals. The device 1000 differs from thedevice 900 of FIG. 9 in that a recess of the device 1000 is shallowerthan a corresponding recess of the device 900, and an insulator formedon a surface of the recess has a single insulation layer with a constantthickness.

Referring to FIG. 10, the semiconductor device 1000 includes asemiconductor substrate 111 having a recess 125 r′. The recess 125 r′has a depth of about 50 Å. A gate electrode 135″ includes a recessedgate electrode portion portion 135 r″ filling the recess 125 r′ of thesemiconductor substrate 111 and a protruding gate electrode portion 135p″ connected to the recessed gate electrode portion portion 135 r″. Theprotruding gate electrode portion 135 p″ includes a bottom gateelectrode portion portion 135 b″ and a main gate electrode portionportion 135 m″. The bottom gate electrode portion portion 135 b″connects to the recessed gate electrode portion portion 135 r″ and has agate length that is less than a width of the recess 125 r′. The maingate electrode portion portion 135 m″ connects to the bottom gateelectrode portion portion 135 b″ and has a gate length that is less thanthe width of the recess 125 r″. An insulator 132′ is interposed betweenthe recessed gate electrode portion portion 135 r″ and the semiconductorsubstrate 111. The insulator 132′ is a single insulation layer with aconstant thickness.

FIGS. 11 through 25 are cross-sectional views showing exemplaryoperations for forming the semiconductor device 800 of FIG. 8. Referringto FIG. 11, a field oxide 113 is formed on a semiconductor substrate 111using, for example, a conventional process. An active region is definedby the field oxide 113. The conventional process may be, for example, ashallow trench isolation (STI) process.

A pad oxide layer 115, a sacrificial nitride layer 117 and an optionalplanarization buffer layer 119 are sequentially formed on thesemiconductor substrate 111. The optional planarization buffer layer 119is formed of a material having an etch selectivity with respect to thesacrificial nitride layer 117, for example, an oxide. The pad oxidelayer 115 may be formed by, for example, a thermal oxidation process ora CVD process. The sacrificial nitride layer 117 and the selectiveplanarization buffer layer 119 may be formed by using, for example, aCVD process. A thickness of the sacrificial nitride layer 117 issubstantially the same as a height of a gate electrode that will beformed in a subsequent process. The optional planarization buffer layer119 will be patterned in a subsequent patterning process, so that thesacrificial nitride layer 117 has a desirable profile (i.e., a gateelectrode having a vertical sidewall may be formed). The optionalplanarization buffer layer 119 may not be formed according to a processcondition.

A photo resist pattern 121 is formed on the optional planarizationbuffer layer 119. The photoresist pattern 121 may be formed by using thefollowing steps: The semiconductor substrate 111 is coated with aphotoresist layer, which is exposed to light by using a predeterminedreticle to form the photoresist pattern 121. The photoresist pattern 121has an opening 123 therein. The opening 123 defines a shape of a gateelectrode that will be formed in a subsequent process.

A gate electrode groove may be formed by using either of two methods asillustrated in FIGS. 12A, 13A and 14A and in FIGS. 12B, 13B and 14B.Referring to FIG. 12A, the optional planarization buffer layer 119 andthe sacrificial nitride layer 117 exposed by the opening 123 in thephotoresist pattern 121 are anisotropically etched until the pad oxidelayer 115 is exposed, thereby forming a main gate electrode groove 125 min the sacrificial nitride layer 117. Referring to FIG. 13A, thephotoresist pattern 121 is removed, and a groove spacer 127 is formed oninner sidewalls of the main gate electrode groove 125 m, thereby forminga main gate electrode center portion groove 125 mc having a shorterwidth than the main gate electrode groove 125 m. Thus, it is possible toform a channel having a short width exceeding resolution capability of aphotolithography process. The groove spacer 127 may be formed of siliconnitride or high-density plasma enhanced tetraethylorthosilicate (TEOS).In order to form the groove spacer 127, a nitride layer or ahigh-density plasma oxide layer is conformally formed on thesemiconductor substrate 111 where the photoresist pattern 121 isremoved, and the nitride or plasma oxide layer is then anisotropicallyetched. Referring to FIG. 14A, the exposed pad oxide layer 115 is etchedusing the groove spacer 127 as an etch mask, thereby forming a bottomgate groove 125 b defined by the pad oxide 115 that exposes a portion ofthe semiconductor substrate 111.

An alternative way of forming a protruding gate electrode groove isexplained by referring to FIGS. 12B, 13B and 14B. Referring to FIG. 12B,the optional planarization buffer layer 119 and a part of thesacrificial nitride layer 117 are anisotropically etched using thephotoresist pattern 121 as an etch mask, thereby forming a main gateelectrode groove 125 m′ that exposes a remaining sacrificial nitridelayer 117. The sacrificial nitride layer 117 is totally etched to exposethe pad oxide pattern 115 in FIG. 12A.

Referring to FIG. 13B, a groove spacer 127′ is formed on sidewalls ofthe main gate electrode groove 125 m′ by using the technique asdescribed above with reference to FIG. 13A, thereby defining a main gateelectrode center portion groove 125 m‘c’. Referring to FIG. 14B, theremaining sacrificial nitride layer 117 and the pad oxide layer 115 areanisotropically etched using the groove spacer 127′ as an etch mask,which exposes a portion of the semiconductor substrate 111. A bottomgate electrode groove 125 b′ is formed through the remaining sacrificialnitride layer 117 and the pad oxide layer 115. The technique of FIGS.12B, 13B and 14B has a difference of height of the bottom gate electrodeportion groove in comparison with the technique of FIGS. 12A, 13A and14A.

Subsequent processes to those shown in FIGS. 12A, 13A and 14A will nowbe described. Referring to FIG. 15, after forming the bottom gateelectrode groove 125 b, the exposed semiconductor substrate 111 isisotropically etched to form a recess 125 r. The recess 125 r may beformed by a wet etching or chemical dry etching. The recess 125 r has awidth that is narrower than a width of the main gate electrode groove125 m of FIG. 12A and wider than a width of the bottom gate electrodegroove 125 b. The contour of the recess 125 r may be a smooth curve,such as a hemispherical or semielliptical shape. The recess 125 r mayhave a thickness greater than about 300 Å.

Referring to FIG. 16, a buffer gate insulator 129 is formed at a surfaceof the recess 125 r and has first thickness. Fort example, the buffergate insulator 129 may be formed by a thermal oxidizing process and witha thicker thickness than a desired thickness of a gate insulator. Achannel ion-implantation process is performed to prevent punch throughand control threshold voltage. In particular, impurity ions areimplanted in the semiconductor substrate 111 under the recess 125 rthrough the buffer gate insulator 129.

Referring to FIG. 17, the buffer gate insulator 129 is anisotropicallyetched. Namely, anisotropic dry etch is carried out on the gateinsulator 129 to remove a portion of the gate insulator 129 on thebottom portion 125 rb of the recess 125 r exposed by the bottom gateelectrode grove 125 b. Also, wet etch can be employed to remove aportion of the gate insulator 129 on the bottom portion 125 rb of therecess 125 r exposed by the bottom gate electrode grove 125 b. As aresult, a buffer gate insulator 129 a remains on sidewalls of the recess125 r, and a bottom portion 125 rb of the recess 125 r having a widthabout the same as the width of the bottom gate electrode groove 125 b isexposed.

Referring to FIG. 18, a gate insulator 131 is formed on the bottomportion 125 rb of the recess 125 r. The gate insulator 131 has athickness less than the thickness of the buffer gate insulator 129 a.The gate insulator 131 may be formed by thermal oxidation.

Although not illustrated, after the buffer gate insulator 129 is formed,a nitride liner may be formed. The nitride liner may be anisotropicallyetched to expose the buffer gate insulator 129. Then, the exposed buffergate insulator 129 may be isotropically etched to expose a portion ofthe semiconductor substrate 111. A thermal oxidation process may then beperformed to form a thinner gate insulator than the buffer gateinsulator on the exposed semiconductor substrate 111.

Referring again to FIG. 18, after forming the gate insulator 131 on thebottom 125 rb of the recess 125 r, a gate electrode material layer 133is formed on the optional planarization buffer layer 119 to fill therecess 125 r, the bottom gate electrode groove 125 b through the padinsulator 115, and the main gate electrode center portion groove 125 mcthrough the sacrificial nitride layer 117. The gate electrode materiallayer 133 may comprise, for example, polysilicon.

Referring to FIG. 19, a planarization process is performed until thesacrificial nitride layer 117 is exposed, thereby forming a recessedgate electrode portion 135 r in the recess 125 r, a bottom gateelectrode portion 135 p in the bottom gate electrode groove 125 b and amain gate electrode center portion 135 c in the main gate electrodecenter portion groove 125 mc. The planarization process may be, forexample, a chemical mechanical polishing (CMP) or an etch back process.The sacrificial nitride layer 117 functions as a planarization stoppinglayer during the planarization process.

Referring to FIG. 20, the sacrificial nitride layer 117 and the groovespacer 127 are removed to expose the main gate electrode center portion135 c. If the groove spacer 127 is formed of a nitride, the sacrificialnitride layer 117 and the groove spacer 127 may be removed using thesame solution, such as phosphoric acid. If the groove spacer 127 isformed of high-density plasma enhanced TEOS, the sacrificial nitridelayer 117 may be removed first by using the phosphoric acid, and thenthe groove spacer 127 may be removed by using a conventional oxide etchsolution.

Referring to FIG. 21, a silicide spacer 137 is formed at sidewalls ofthe exposed main gate electrode center portion 135 c. The gate electrodecenter portion 135 c and the silicide spacer 137 form a main gateelectrode portion 135 m, and the main gate electrode portion 135 m andthe bottom gate electrode portion 135 b form a protruding gate electrodeportion 135 p. The protruding gate electrode portion 135 p and therecess gate electrode portion 135 r form a gate electrode 135 having ashape of an anchor.

In order to form the silicide spacer 137, a metal silicide layer may beformed by a conventional method, such as CVD, and etched back. A metalsilicide layer may be conformally formed on the gate electrode center135 c and the pad oxide layer 115 and the metal silicide layer is etchedback until the pad oxide layer 115 is exposed. As a result, a metalsilicide layer remains on sidewalls of the main gate electrode centerportion portion 135 c as a spacer. The silicide spacer 137 may be formedof, for example, tungsten silicide or cobalt silicide. A barrier layermay be formed before forming the silicide spacer 137. The barrier layermay be formed of, for example, titanium silicide.

Referring to FIG. 22, after forming the silicide spacer 137, the exposedpad oxide layer 115 is etched back such that the portion of pad oxidelayer 115 that does not underlie the suicide spacer 137 is removed toexpose the semiconductor substrate 111 and a portion 115 a of the padoxide layer 115 a remains under the silicide spacer 137. A lowconcentration impurity-doped region 139 may then be formed into theexposed semiconductor substrate 111 adjacent to the silicide spacer 137by implanting impurity ions of low concentration to form a lightly dopeddrain (LDD) region. The low concentration impurity-doped region 139preferably has a depth that is substantially the same as that of therecess 125 r. Depths of the low concentration impurity-doped region anda subsequent high concentration impurity-doped region may be dependenton the depth of the recess 125 r.

Referring to FIG. 23, a gate insulation spacer 141 is formed onsidewalls of the remaining pad oxide 115 a and the silicide spacer 137.The gate insulation spacer 141 may be formed from a nitride material. Inorder to form the gate insulation spacer 141, a nitride layer may beconformally formed and etched back.

Referring to FIG. 24, impurity ions of high concentration are implantedinto the semiconductor substrate 111 adjacent to the gate insulationspacer 141 using the gate insulation spacer 141 as an ion-implantationmask, thereby forming a high concentration impurity-doped regions 143.The high concentration impurity-doped regions 143 and the lowconcentration impurity-doped regions 139 serve as source/drain regions.The high concentration impurity-doped region 143 may be more deeplyformed than the low concentration impurity-doped region 139.

The ion-implantation process for formation of the low concentrationimpurity-doped region may be omitted. That is, if the recess 125 r isformed to a depth of about 800 Å, the ion-implantation process forformation of the low concentration impurity-doped region may be omitted.In such a case, when the ion-implantation process for formation of thehigh concentration impurity-doped region is performed, the lowconcentration impurity-doped region may be simultaneously formedaccording to a concentration profile. However, the ion-implantationprocess for formation of the low concentration impurity-doped region maybe omitted even though the recess has a different depth.

Referring to FIG. 25, a metal layer 145 comprising cobalt, titanium ornickel is formed on the semiconductor substrate 111 on the exposedsemiconductor substrate 111, on the gate insulation spacer 141 and on anupper surface of the gate electrode 135. The metal layer 145 isthermally treated to form a silicide layer 147 on the high concentrationimpurity-doped region 143 and on an upper surface of the gate electrode135.

Operations for forming the semiconductor device 900 of FIG. 9 will nowbe explained with reference to FIGS. 26 through 31. Discussion of likeoperations described above with reference to FIGS. 10 through 25 will beomitted. Referring to FIG. 26, a gate electrode center groove 125 m‘c’and a bottom gate electrode groove 125 b are formed as described abovewith reference to FIGS. 11, 12B, 13B and 14B. A groove spacer 127′ isremoved to form a main gate electrode groove 125 m and a bottom gateelectrode groove 125 b. The exposed semiconductor substrate 111 isisotropically etched to form a recess 125 r. A width of the recess 125 ris less than that of the main gate pattern groove 125 m but greater thanthat of the bottom gate electrode groove 125 b.

Referring to FIG. 27, a buffer gate insulator 129 of a first thicknessis formed on a surface of the recess 125 r using a thermal oxidationprocess. A nitride liner 130 is formed on the semiconductor substrate111 where the buffer gate insulator 129 is formed, i.e., on the buffergate insulator 129, on inner walls of the gate electrode groove 125 andon the planarization buffer layer 119. The nitride liner 130 may beformed using a CVD process.

Referring to FIG. 28, an etch back process is performed to remove thenitride liner 130 on the bottom 125 rb of the recess 125 r and expose apart of the buffer gate insulator 129. The exposed part of the buffergate insulator 129 is etched to expose the bottom 125 rb of the recess125 r of the semiconductor substrate 111. The buffer gate insulator onboth sidewalls 125 rs of the recess 125 r is not removed. The removal ofthe exposed part of the buffer gate insulator 129 can be done by dryetch or wet etch. A gate insulator 131 is formed on the bottom 125 rb ofthe recess 125 r, and is relatively thinner than the buffer gateinsulator 129. The gate insulator 131 may be formed by using a thermaloxidation process.

Referring to FIG. 29, a conductive material, such as polysilicon, isformed to fill the gate electrode groove 125 and planarized down to thesacrificial nitride layer 117, thereby forming a gate electrode 135′.The gate electrode 135′ includes a recessed gate electrode portion 135r′ filling the recess 125 r, a bottom gate electrode portion 135 b′filling the bottom gate electrode groove 125 b and a main gate electrodeportion 135 m′ filling the main gate electrode portion groove 125 m.

Referring to FIG. 30, the sacrificial nitride layer 117 is removed usingan etch solution, such as phosphoric acid. The nitride liner on thesidewalls of the main gate electrode portion 135 m′ and the remainingsacrificial nitride layer 117 may be simultaneously removed. Thus, abottom surface of the main gate electrode portion 135 m′ may bepartially exposed. Using operations described above, a low concentrationimpurity-doped region 139 may be formed by implanting impurity ions oflow concentration using the gate electrode 135′ as an ion-implantationmask.

Referring to FIG. 31, a nitride layer is formed on the resultantstructure having the low concentration impurity-doped region 139. Anetch back process is performed with respect to the nitride layer to forma gate insulation spacer 141. The etch back process is performed untilthe buffer oxide layer 115 is exposed. An etch back process is thenperformed with respect to the exposed buffer oxide layer 115, therebyexposing the semiconductor substrate 111 that does not underlie the gateinsulation spacer 141 while leaving a buffer oxide layer 115 a under thegate insulation spacer 141. A high concentration impurity-doped region143 is formed by implanting impurity ions of high concentration usingthe gate insulation spacer 141 as an ion-implantation mask.

Operations for forming the semiconductor device 1000 of FIG. 10 will nowbe explained with reference to FIGS. 32 through 36. Discussion of likeoperations described above will be omitted. Referring to FIG. 32,described above with reference to FIGS. 11, 12A, 13A and 14A, afterforming a main gate electrode center portion groove 125 mc and a bottomgate electrode groove 125 b, a groove spacer 127 is removed, therebyforming a main gate electrode groove 125 m and a bottom gate electrodegroove 125 b. The exposed semiconductor substrate 111 is isotropicallyetched to form a recess 125 r′. The recess 125 r′ may have a veryshallow depth, e.g., about 50 Å. The recess part 125 r′ has a width notexceeding that of the main gate electrode groove 125 m.

Referring to FIG. 33, a gate insulator 132′ is formed on a surface ofthe recess 125 r′. The gate insulator 132′ may be formed by using athermal oxidation process. The gate insulator 132′ has a constantthickness, in contrast to gate insulators of the aforedescribedembodiments. A gate electrode material 133 is formed on theplanarization buffer layer 119, thereby filling the gate electrodegroove 125.

Referring to FIG. 34, a planarization process is performed until thesacrificial nitride layer 117 is exposed, thereby forming a gateelectrode 135″. The gate electrode 135″ includes a recessed gateelectrode portion 135 r″ filling the recess 125 r′, a bottom gateelectrode portion 135 b″ filling the bottom gate electrode groove 125 b,and a main gate electrode portion 135 m″ filling the main gate electrodegroove 125 m.

Referring to FIG. 35, the sacrificial nitride layer 117 is removed,thereby exposing the pad oxide layer 115 and the main gate electrodeportion portion 135 m″ of the gate electrode 135″. A low concentrationimpurity-doped region 139 is formed using an ion-implantation process.

Referring to FIG. 36, a nitride layer is formed on the resultantstricture where the low concentration impurity-doped region 139 isformed. The nitride layer is etched back to form a gate insulationspacer 141. An etch back process is performed until the buffer oxidelayer 115 is exposed. The exposed buffer oxide layer 115 is etched backto expose the portion of the semiconductor substrate 111 that does notunderlie the gate insulation spacer 141, while the buffer oxide 115remains under the gate insulation spacer 141. A high concentrationimpurity-doped region 143 is formed by implanting impurity ions at highconcentration using the gate insulation spacer 141 as anion-implantation mask.

The recess part 125 r′ is shallowly formed. The recess 125 in theembodiments described above with reference to FIGS. 11 through 25 may belikewise shallowly formed, i.e., with a shallow depth of about 50 Å.This will be briefly explained by referring to FIGS. 37 through 40.

Referring to FIG. 37, a main gate electrode center portion groove 125 mcand a bottom gate electrode groove 125 b are formed by using, forexample, the operations described above with reference to FIGS. 11, 12A,13A, 14A and 15. An exposed semiconductor substrate 111 is isotropicallyetched to form a shallow recess 125 r′. The recess 125 r′ may have adepth of about 50 Å. A thermal oxidation process is then performed toform a gate insulator 132 on the shallow recess 125 r′. A gate electrodematerial 133 is formed on the selective planarization buffer layer 119,thereby filling the recess 125 r′, the bottom gate electrode groove 125b through the pad oxide layer 115 and the main gate electrode centerportion groove 12 mc through the sacrificial nitride layer 117.

Referring to FIG. 38, a planarization process is performed until thesacrificial nitride layer 117 is exposed, thereby forming a recessedgate electrode portion 135 r′″ filling the recess 125 r′, a bottom gateelectrode portion 135 b′″ filling the bottom gate electrode groove 125 band a main gate electrode center portion portion 135 c′″ filling themain gate electrode center portion 125 mc.

Referring to FIG. 39, the sacrificial nitride layer 117 and the groovespacer 127 are removed to expose the main gate electrode center portion135 c. A silicide spacer 137 is formed on both sidewalls of the exposedmain gate electrode center portion 135 c. Thus, a gate electrode 135′″is formed. In particular, the main gate electrode center portion 135 c′″and the silicide spacer 137 form the main gate electrode portion 135m′″, and the bottom gate electrode portion 135 b′″ and the main gateelectrode portion “ ” 135 m form a protruding gate electrode portion 135p′″. The protruding gate electrode portion 135 p′″ and the recessed gateelectrode portion 135 r′″ form a gate electrode 135′″ having a shape ofan anchor.

After forming the silicide spacer 137, the exposed pad oxide layer 115is etched back, thereby removing the portion of the pad oxide layer 115that does not underlie the silicide spacer 137 and exposing thesemiconductor substrate 111. A portion 115 a of the pad oxide layerremains under the silicide spacer 137. A low concentrationimpurity-doped region 139 is formed in the exposed semiconductorsubstrate 111 adjacent to the silicide spacer 137 by implanting impurityions of low concentration, thus forming an LDD region.

Referring to FIG. 40, a gate insulation spacer 141 is formed onsidewalls of the remaining pad oxide layer 115 a and the silicide spacer137. A high concentration impurity-doped region 143 is formed in thesemiconductor substrate 111 adjacent to the gate insulation spacer 141by implanting impurity ions at a high concentration. The highconcentration impurity-doped region 143 and the low concentration dopedregion 139 serve as source/drain regions.

According to some embodiments of the present invention, a recess isformed in a semiconductor substrate. A thin gate insulator is formed ona bottom of the recess and a thick gate insulator is formed on sidewallsthereof. Thus, parasitic capacitance between the semiconductor substrateand a gate electrode in the recess may be reduced part due to the thickgate insulator on the sidewalls of the recess. Because an effective gateinsulator is formed on the bottom of the recess, a semiconductor devicehaving a short channel and exhibiting high speed and/or low powerconsumption can be provided. Additionally, a silicide layer may beformed on sidewalls of a gate electrode, thereby decreasing a resistanceof a semiconductor device.

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms areemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being set forthin the following claims.

1. A method of forming a semiconductor device, comprising: forming aninsulation layer on a substrate; forming a sacrificial layer on theinsulation layer; forming an opening through the sacrificial layer andthe insulation layer to expose an active region of the substrate;removing a portion of the active region to form a recess therein thathas a greater width than the opening through the sacrificial layer andthe insulation layer; forming a gate insulator in the recess; forming agate electrode extending through the sacrificial layer and theinsulation layer and into the recess, the gate electrode comprising afirst portion on the gate insulator in the recess and a secondreduced-width portion extending from the first portion; removing thesacrificial layer to expose a sidewall of the gate electrode outside ofthe recess; forming an insulating spacer on the exposed sidewall of thegate electrode; and forming a source/drain region in the substrateadjacent the recess.
 2. The method as claimed in claim 1, wherein theforming a gate insulator comprises: forming a first insulating layerhaving a first thickness in the recess; removing a portion of the firstinsulating layer on a bottom of the recess to expose a bottom of therecess while leaving a portion of the first insulating layer on asidewall of the recess; and forming a second insulation layer on theexposed bottom of the recess, the second insulation layer having asecond thickness less than the first thickness.
 3. The method of claim2, wherein removing a portion of the first insulating layer is precededby forming a nitride liner on the first insulation layer, whereinremoving a portion of the first insulating layer on a bottom of therecess to expose a bottom of the recess while leaving a portion of thefirst insulating layer on a sidewall of the recess comprises removingportions of the first insulating layer and the nitride liner on thebottom of the recess to expose the bottom of the recess while leavingportions of the first insulating layer and the nitride liner on asidewall of the recess.
 4. The method of claim 1, wherein removing aportion of the active region to form a recess therein is preceded byforming a sidewall spacer on a sidewall of the sacrificial layer and theinsulation layer, and wherein removing a portion of the exposed activeregion comprises etching the exposed active region using the sidewallspacer as an etching mask.
 5. The method of claim 4, wherein forming agate electrode comprises depositing a conductive material through anopening defined by the sidewall spacer.
 6. The method of claim 1,wherein forming an opening comprises: removing a portion of thesacrificial layer overlying the active region to expose the insulationlayer; forming a sidewall spacer on a sidewall of the sacrificial layer;and etching an exposed portion of the insulation layer using thesidewall spacer as an etching mask to expose a portion of the activeregion.
 7. The method of claim 1, wherein forming an opening comprises:removing a portion of the sacrificial layer overlying the active regionwithout exposing the insulation layer; forming a sidewall spacer on asidewall of the sacrificial layer; and etching through portions of thesacrificial layer and the insulation layer using the sidewall spacer asan etching mask to expose a portion of the active region.
 8. The methodof claim 7, wherein forming a gate electrode comprises: removing thesidewall spacer to form an enlarged opening through at least one portionof the sacrificial layer; and depositing conductive material into theenlarged opening.
 9. The method of claim 1: wherein forming aninsulating spacer comprises forming a first insulating spacer on theexposed sidewall of the gate electrode; wherein forming a source/drainregion comprises implanting ions into the substrate using the firstinsulating spacer as an implantation mask; wherein forming an insulatingspacer further comprises forming a second insulating spacer on the firstinsulating spacer; and wherein forming a source/drain region comprisesimplanting ions into the substrate using the second insulating spacer asan implantation mask to thereby form a source/drain region comprisinglighter and heavier doped portions.
 10. The method of claim 1, furthercomprising forming a planarization buffer layer on the sacrificiallayer, wherein forming an opening through the sacrificial layer and theinsulation layer to expose an active region of the substrate comprisesforming an opening through the planarization buffer layer, thesacrificial layer and the insulation layer, and wherein forming a gateelectrode extending through the sacrificial layer and the insulationlayer and into the recess comprises: depositing a conductive material inthe opening through the planarization buffer layer, the sacrificiallayer and the insulation layer; and planarizing the substrate to removethe planarization buffer layer and a portion of the deposited conductivematerial overlying the sacrificial layer.
 11. The method of claim 1,wherein removing a portion of the active region comprises performing anisotropic etching process.
 12. The method of claim 11, wherein therecess has a rounded bottom surface formed by the isotropic etchingprocess.
 13. The method of claim 1, further comprising forming asilicide spacer on the insulation layer before forming the insulatingspacer, the silicide spacer being in contact with the exposed thesidewall of the gate electrode.